Sleep-related assessments and interventions are increasingly tested in pediatric, psychological, and educational practice. However, the developmental science basis for practice is quite limited. Studies of naturally-occurring sleep have identified sleep deficit variables, but have not placed them in a clear dimensional structure; they have implied but not sufficiently established that sleep affects children's social development; and they have suggested but not established a mechanism for how sleep might shape social development, via effects on growth in self-regulation. To better define the dimensions of sleep deficits and chart the role of sleep deficits in children's social development outcomes, the proposed project longitudinally studies children at ages 2 1/2 , 3, and 3 1/2, a period of relatively intense development in both sleep and self- regulation that sets the stage for later peer and academic success. Children's sleep deficits will be assessed by actigraphs and parent diary, and sleep problems by a clinically-used parent-report scale; self-regulation by laboratory tasks; and adjustment by parent and secondary caregiver reports. Family context, including stressors and parenting, will be assessed at the same times, with questionnaire, interview, and observational methods, and also with the topically relevant, novel method of observing the prelude to the child's bedtime. The following 7 hypotheses are tested: 1) Actigraphic and parent diary measures of child sleep deficit will yield 3, psychometrically sound, separable factors: short amount, night-to-night variability in timing and amount, and within-night fragmentation of sleep, and these factors will both form one 2nd-order factor at each age and apply at all 3 ages. 2) Children will show moderate levels of continuity on the sleep deficit factors. 3) Family context will relate to child sleep to moderate degrees. 4) Sleep deficit factors will be associated with child social adjustment measures both within and across ages, even controlling for family context variables that are also associated with child adjustment. 5) Sleep deficits will also be associated with child self-regulation and its growth, even controlling for family context. 6) Assuming hypotheses 4 and 5 will be supported, child growth in self-regulation will explain effects of sleep on child social development. 7) And as a secondary hypothesis, we expect that temperamental unmanageability and cognitive-verbal skills will each moderate the linkage between sleep deficits and social development outcomes.